The present invention relates generally to masks used in making semiconductors devices, and more particularly to phase shift masks.
In semiconductor device manufacturing, features and geometric patterns are created on semiconductor wafers using conventional optical photolithography. Typically, optical photolithography involves projecting or transmitting light through a pattern made of optically opaque areas and optically clear areas on a mask.
A prior art mask 100 used to pattern a semiconductor wafer is shown in FIG. 1. A transparent substrate 102 comprising silicon quartz, for example, is provided. An opaque layer 104 is deposited over the substrate 102. The opaque layer 104 typically comprises chrome, for example. The opaque layer 104 is patterned with the desired pattern so that light may pass through holes 106 in the opaque layer 104 when the mask is used to pattern a semiconductor wafer. This type of mask 100 is often referred to as a binary chrome-on-glass mask.
The optically opaque areas 104 of the pattern block the light, thereby casting shadows and creating dark areas, while the optically clear areas 106 allow the light to pass, thereby creating light areas. Once the light areas and dark areas are formed, they are projected onto and through a lens and subsequently onto a semiconductor substrate. However, because of increased semiconductor device complexity that results in increased pattern complexity, and increased pattern packing density on the mask, the distance between two of the opaque areas 104 is continually being decreased.
By decreasing the distances between the opaque areas 104, small apertures are formed which diffract the light that passes through the apertures. The diffracted light results in effects that tend to spread or to bend the light as it passes through the mask 100 so that the space between the two opaque areas is not resolved, therefore making diffraction a severe limiting factor for optical photolithography. More particularly, imaging is degraded because light from clear areas 106 on the mask 100 is diffracted into regions that ideally would be completely dark. The nominally dark region has light diffracted into it from the space on either side.
A conventional method of dealing with diffraction effects in optical photolithography is achieved by using a phase shift mask (PSM), which replaces the previously discussed mask. Generally, with light being thought of as a wave, the term xe2x80x9cphase shiftingxe2x80x9d refers to a change in timing or a shift in waveform of a regular sinusoidal pattern of light waves that propagate through a transparent material. Typically, phase shifting is achieved by passing light through areas of a transparent material of either differing thicknesses or through materials with different refractive indexes, thereby changing the phase or the periodic pattern of the light wave.
Phase shift masks attempt to reduce diffraction effects by combining both diffracted light and phase-shifted light so that constructive and destructive interference takes place. The desired result of using a phase shift mask is that a summation of the constructive and destructive interference results in improved resolution and improved depth of focus.
One particular type of phase shift mask is an alternating phase shift mask. An example of an alternating phase shift mask 200 is shown in FIG. 2. A transparent substrate 202 comprising silicon quartz, for example, is provided. An opaque layer 204 is deposited over the substrate 202. The opaque layer 204 typically comprises chrome, for example. The opaque layer 204 is patterned with a desired patterned so that light may pass through holes 206 in the opaque layer 204 when the mask is used to pattern a semiconductor wafer. A phase shifting material 208 is deposited over the opaque layer 204. The phase shifting material 208 is patterned, and portions are removed to leave transparent regions 206 and transparent phase shifted regions 209 through which light can pass through to illuminate and pattern a semiconductor wafer.
In an alternating phase shift mask 200, alternating clear regions 209 cause the light to be phase-shifted 180 degrees, so that light diffracted into the nominally dark area from the clear area 209L to the left will interfere destructively with light diffracted from the right clear area 209R. This destructive interference of diffracted light results in improved image contrast, as shown in FIG. 3.
FIG. 3 illustrates a comparison of the light intensity 110 of a conventional mask 100 such as the one shown in FIG. 1 with the light intensity 210 of an alternating phase shift mask 200 shown in FIG. 2. The higher slope of the light intensity 210 curve of the alternating phase shift mask 200 indicates a higher resolution and improved image contrast compared to the light intensity 110 curve of a conventional mask 100.
The alternating phase shift mask 200 includes clear area 206 (0 degrees) adjacent clear area 209 (shifted by 180 degrees). These phase shifted clear areas 206/209 may interfere destructively, resulting in the light intensity distribution profile shown in FIG. 4 at 212. This optical image may change the topology of the resist pattern, requiring further process steps or different types of alternating phase shift masks be used to prevent or minimize this effect. For example, excess resist resulting from phase conflicts at line ends on the semiconductor wafer are often trimmed away in a second exposure step. Prior art alternating phase shift masks utilize one phase edge (region where clear area 206 abuts phase shifted clear area 208) to pattern a feature.
The present invention achieves technical advantages as an alternating phase-shift mask and method of manufacturing thereof having improved resolution. Assist phase edges are positioned on either side of a phase edge to enhance the ultimate resolution of an alternating phase-shift mask.
A preferred embodiment of a method of manufacturing a phase shift mask includes providing a transparent substrate, patterning the substrate with a geometric pattern, the geometric pattern including a phase edge and at least one assist edge proximate the phase edge, wherein the assist edge is adapted to improve the resolution of the phase edge.
Another preferred embodiment of a phase shift mask includes a substrate that permits light to pass through, the substrate comprising a geometric pattern, the geometric pattern including a phase edge and at least one assist edge proximate the phase edge.
Further disclosed is a preferred embodiment of a method of manufacturing a semiconductor device, comprising providing a semiconductor wafer, depositing a resist layer on the semiconductor wafer, illuminating portions of the resist layer to leave at least a first illuminated resist portion, a first non-illuminated resist portion adjacent the first illuminated resist portion, a second illuminated resist portion adjacent the first non-illuminated resist portion, a second non-illuminated resist portion adjacent the second illuminated resist portion, and a third illuminated resist portion adjacent the second non-illuminated resist portion. The method includes removing at least the first, second and third illuminated resist portions, and removing at least the first non-illuminated resist portion.
Advantages of the embodiments of the present invention include enhancing the resolution of an alternating phase shift mask. The assist edges are positioned at a pre-determined distance from the main phase edge in order to improve the aerial image contrast. Smaller feature sizes may be manufactured on a semiconductor wafer than with prior art phase shift masks in accordance with embodiments of the present invention.